Returns a copy of this {@code ZonedDateTime} with the specified period in days added.
This operates on the local time-line, {@link LocalDateTime#plusDays(long) adding days} to the local date-time.This is then converted back to a {@code ZonedDateTime}, using the zone ID to obtain the offset.
When converting back to {@code ZonedDateTime}, if the local date-time is in an overlap, then the offset will be retained if possible, otherwise the earlier offset will be used. If in a gap, the local date-time will be adjusted forward by the length of the gap.
This instance is immutable and unaffected by this method call.
@param days the days to add, may be negative
@return a {@code ZonedDateTime} based on this date-time with the days added, not null
@throws DateTimeException if the result exceeds the supported date range